Human hepatitis C (HCV) is a major public health problem with an estimated 200 million persons worldwide infected. The number of new infections per year in the United States is estimated to be about 25,000 in 2001. This number has declined from an estimated 240,000 new cases per year in the 1980's due to blood donor screening. Nevertheless, an estimated 3.9 million (1.8%) Americans have been infected with HCV, of whom 2.7 million are chronically infected. Hepatitis C shows significant genetic variation in worldwide populations, evidence of its frequent rates of mutation and rapid evolution. There are six basic genotypes of HCV, with 15 recorded subtypes, which vary in prevalence across different regions of the world. Each of these major genotypes may differ significantly in their biological effects—in terms of replication, mutation rates, type and severity of liver damage, and detection and treatment options—however, these differences are not yet clearly understood.
There is currently no vaccine against HCV and available drug therapy, including ribavirin and interferon, is only partially effective. It is estimated that 75-85% of infected persons will develop a chronic infection, with 70% of chronically infected persons expected to develop chronic liver 5 disease including hepatocellular carcinoma. Chronic HCV related liver disease is a leading indication for liver transplant.
Although a human hepatitis B vaccine has been available since 1982, it is estimated that 350 million persons worldwide are chronically infected with HBV. While the number of new infections per year in the United States has declined from an average of 260,000 in the 1980s to about 78,000 in 2001, there are an estimated 1.25 million hepatitis B carriers, defined as persons positive for hepatitis B surface antigen (HBsAg) for more than 6 months. Such carriers of HBV are at increased risk for developing cirrhosis, hepatic decompensation, and hepatocellular carcinoma. Although most carriers do not develop hepatic complications from chronic hepatitis B, 15% to 40% will develop serious sequalae during their lifetime, and death from chronic liver disease occurs in 15-25% of chronically infected persons.
There is a need for improved therapeutic agents effective in patients suffering from HBV and/or HCV infection, especially chronic infection, which together are estimated to account for 75% of all cases of liver disease around the world. There is also an extreme need for prophylactic methods and agents effective against HCV.
Nucleic acids (e.g., DNA, RNA, hybrid, heteroduplex, and modified nucleic acids) have come to be recognized as extremely valuable agents with significant and varied biological activities, including their use as therapeutic moieties in the prevention and/or treatment of disease states in man and animals. For example, oligonucleotides acting through antisense mechanisms are designed to hybridize to target mRNAs, thereby modulating the activity of the mRNA. Another approach to the utilization of nucleic acids as therapeutics is designed to take advantage of triplex or triple strand formation, in which a single-stranded oligomer (e.g., DNA or RNA) is designed to bind to a double-stranded DNA target to produce a desired result, e.g., inhibition of transcription from the DNA target. Yet another approach to the utilization of nucleic acids as therapeutics is designed to take advantage of ribozymes, in which a structured RNA or a modified oligomer is designed to bind to an RNA or a double-stranded DNA target to produce a desired result, e.g., targeted cleavage of RNA or the DNA target and thus inhibiting its expression. Nucleic acids may also be used as immunizing agents, e.g., by introducing DNA molecules into the tissues or cells of an organism that express proteins capable of eliciting an immune response. Nucleic acids may also be engineered to encode an RNA with antisense, ribozyme, or triplex activities, or to produce RNA that is translated to produce protein(s) that have biological function.
More recently, the phenomenon of RNAi or double-stranded RNA (dsRNA)-mediated gene silencing has been recognized, whereby dsRNA complementary to a region of a target gene in a cell or organism inhibits expression of the target gene (see, e.g., WO 99/32619, published 1 Jul. 1999, Fire et al.; and U.S. Pat. No. 6,506,559: “Genetic Inhibition by Double-Stranded RNA;” WO 00/63364: “Methods and Compositions for Inhibiting the Function of Polynucleotide Sequences,” Pachuk and Satishchandran; and U.S. Ser. No. 60/419,532, filed Oct. 18, 2002). dsRNA-mediated gene silencing, utilizing compositions providing an at least partially double-stranded dsRNA, is expected to provide extremely valuable therapeutic and/or prophylactic agents against viral infection, including HBV and/or HCV, including in the extremely difficult problem of chronic HBV and/or HCV infection.